1. Field of the Invention
The present invention relates generally to a solid state camera, and more particularly to a solid state television camera which uses a semiconductor element, for example, a charge coupled device as an image sensor.
2. Description of the Prior Art
Solid state television cameras having been proposed in the prior art which use charge coupled devices (hereinafter referred to as CCDs) as solid state image sensors.
The solid state image sensor or CCD used in the solid state television camera is shown in FIG. 1 generally by numeral 1. The CCD 1 is made of a photo sensitive array 1A which consists of a plurality of picture elements 4 arranged on one surface of a semiconductor substrate in horizontal and vertical directions and onto which an object is projected; a temporary storage array 1B which is substantially the same as the photo sensitive array 1A except that it is shielded optically and stores carriers corresponding to received light information of an object; and a read-out resistor 1C which reads out carriers during each horizontal scanning period from the temporary storage array 1B. In FIG. 1, an output terminal 5 is extended from the read-out register 1C and 6 channel stoppers, respectively.
Since, in the CCD 1, it is rather difficult to form the crystal of the semiconductor uniform over a predetermined area, crystal defects are produced locally, and electric charges are apt to be produced at those defects by thermal causes. As a result, when the camera using the above CCD picks up an object whose image is projected on the CCD, noise appears in the output signal at non-illuminated portions thereof, where the current is abnormally large. In this case, this noise exceeds the white level, so that it appears in the reproduced picture and is undesirable.
In order to remove such noise, it is sufficient to provide a defect compensating circuit and to control a sampling hold circuit 7 therein to which a read-out or video signal from the CCD 1 is supplied as shown in FIG. 2. Assuming a video signal S.sub.A, derived from the CCD1, delivered to the output erminal 5 and fed to an amplifier 13, is a rectangular wave as shown in FIG. 3A. Since the level of a noise signal S.sub.N corresponding to the crystal defect is higher than the white level L.sub.W, the noise signal S.sub.N can be shown in FIG. 3A by the dotted line. Thus, a level detecting circuit 8 for the video signal S.sub.A is provided in the noise eliminating circuit 10. In this case, the detecting level of the circuit 8 is set at a desired level, for example, level L.sub.N which is higher than the white level L.sub.W as shown in FIG. 3A. An output S.sub.D obtained from the detecting circuit 8, when the level of a signal fed thereto exceeds the detecting level L.sub.N, is supplied to a gate circuit 9, which is supplied with a sampling signal S.sub.S, so as to control the gate circuit 9.
In other words, if the gate circuit 9 is turned "off" by the detected output S.sub.D from the detecting circuit 8, a signal S.sub.S1 corresponding to the inherent sampling signal S.sub.S (refer to FIG. 3B) is eliminated from the output of the gate circuit 9 (accordingly, the final output from the gate circuit 9 is a signal S.sub.S, shown in FIG. 3D). When the signal S.sub.S1 is eliminated, no sampling operation is carried out. As a result, an output from the sampling hold circuit 7 becomes a waveform S.sub.H as shown in FIG. 3E.
If, however, the above defect compensating operation is not carried out, an output S.sub.H, based upon the corresponding sampling signal S.sub.S1 is obtained from the sampling hold circuit 7 as shown in FIG. 3C, with the result that the reproduced picture is deteriorated in quality.
Since, in the above defect compensating circuit 10, the output S.sub.D from the level detecting circuit 8 is supplied to the gate circuit 9 to control the sampling signal S.sub.S, the detecting operation thereof is delayed due to the response time .tau. of the detecting circuit 8. For this reason, in the noise eliminating circuit 10, there is provided at the former stage of the sampling hold circuit 7 a delay circuit or line 11 whose delay time is at least as great as the response time .tau.. This delay line 11 delays the video signal S.sub.A by a predetermined time and applies the delayed video signal to the sampling hold circuit 7.
As described above, it is necessary to provide the delay line 11 in the noise eliminating circuit 10, so that the waveform of the video signal S.sub.A is distorted by the delay line 11 and hence the solid state camera is deteriorated in fidelity.
Further, since the delay line 11 is costly, the camera becomes more costly to produce.